What appears to be a change in the treatment of some asylum seekers may be aimed at easing pressure on the facilities of Royal Darwin Hospital.

An advocacy group for children in detention says an Immigration Department practice of sending all pregnant asylum seekers from Christmas Island to detention centres in Darwin has changed.

Child advocacy group Chilout says the during the past few weeks about 17 pregnant women have been sent to the Inverbrackie detention centre in Adelaide and Melbourne Immigration Transit Accommodation, rather than to Darwin.

Spokeswoman Sophie Peer says the change should ease the burden on Royal Darwin Hospital (RDH).

"Certainly there is pressure on Royal Darwin Hospital," she said.

"They simply cannot cope ... something that the hospital staff have been talking about for many months."

Ms Peer says about 75 pregnant women are being held in detention on the Australian mainland, and most are still in Darwin.

Chilout has urged the Federal Government to pay for the employment of two more midwives at RDH.